Conditional Advertising in Multihomed Customer Networks

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• Customer edge routers should announce the whole customer address space into BGP.

• The static route covering the whole customer address should point to the core of the customer network, not to null 0.

• Customer edge router revokes the BGP announcement of customer address space if the edge router loses connectivity with the customer core.

As a rule of thumb, the customer should announce addresses as large as possible (the larger the address space that can be aggregated, the better). The BGP advertisement is configured on the customer edge routers using the network command. Route advertisement is conditioned by the appearance of a corresponding network or subnet in the routing table of the edge router. If the network or subnet is manually entered into the routing table by configuring a static route to "null 0," the condition is always true because the static route is always there, and the BGP advertisement is always performed.

If the customer edge router loses connectivity to the rest of the customer network but is still connected to the ISP network, BGP advertisement must cease. In this case, BGP advertisement can be stopped if BGP advertisements are bound to the reachability status of a specific subnet in the core of the customer network, according to the customer IGP.

The problem with using a static route to null 0 is that it conditions the network statement in the BGP configuration so that BGP always advertises the route. If the customer edge router loses connectivity with the rest of the customer network, the router continues to advertise the entire customer address space. The ISP network receives a valid route from the customer edge router. Traffic is sent to this router, but because it has lost connectivity with the rest of the network, the traffic is dropped (routed to the null 0 interface using the static route).

5-52 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

Conditional Advertising in Multihomed Customer Networks (Cont.)

© 2004 Cisco Systems, Ii

© 2004 Cisco Systems, Ii

In this example, the customer network uses the address space 13.5.0.0/16. The address space is further subnetted at the customer site. One of the subnets (subnet 13.5.1.0/24) is identified as being a central part of the customer core network.

The customer edge routers participate in the IGP routing of the customer. This participation means that these routers have information about which of the subnets within the address space 13.5.0.0/16 are currently reachable. If these subnets are available, there is an explicit route to each of them. If any of the subnets go down, or the path toward them goes down, the route to that subnet is removed from the routing table.

The BGP advertisement in each of the customer edge routers is configured to advertise the full address space that is used by the customer. When this route is advertised by the customer edge routers, the ISP network, and thus the rest of the Internet, see the complete address space of the customer as one single route, 13.5.0.0/16.

Advertisement of the customer address space by BGP is conditioned by the appearance of the static route, ip route 13.5.0.0 255.255.0.0 13.5.1.1. If the static route is valid, then the BGP route 13.5.0.0/16 is advertised. The static route is a recursive route, which means that the router takes another look in the routing table for the address 13.5.1.1 before determining what to do with the static route. The idea is that 13.5.1.1 is reachable via the IGP. The subnet 13.5.1.0/24 is announced by the IGP. If this subnet is reachable by the edge router, then the static route to 13.5.0.0/16 is valid. If there is no route to 13.5.1.1, then the static route is invalid.

Note The condition, whether or not to advertise the entire customer address space 13.5.0.0/16, is controlled by the IGP reachability of a single subnet, 13.5.1.0/24.

The IGP configuration also includes origination of the default route by both edge routers.

Copyright © 2004, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP 5-53

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

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